Can we make Ragwort safer for horses?

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Ragwort is often seen as a dangerous weed, but this plant has a little-known relationship with ants and aphids which could help us reduce its toxicity and let us live peacefully beside it. 

Ragwort has a reputation among horse owners for being a dangerous weed – their bright yellow flowers litter the British countryside every year, and their abundance at Knepp led to anger among local residents. But this plant is native to the UK, and horses have lived alongside it for thousands of years, so is the concern justified? 

How does ragwort affect horses?

The toxic alkaloid chemicals which ragwort uses to fend off attack causes liver poisoning in horses, accumulating over time, according to the British Horse Society. Some sources suggest that horses might need to consume 7% – 20% of body weight to be affected, and recoveries from acute poisoning have been recorded in the medical literature. The BHS suggests that horses won’t choose to eat ragwort in the field, due to its bitter taste. The danger comes mostly from its presence in hay – when ragwort has wilted or dried, the bitter taste is lost, but not its liver-damaging alkaloid toxins. 

Sheep grazing on a field in North Somerset

Fields tend to be quite smooth unless they’re grazed by larger livestock or host ant colonies 

A tale of tussocks and tents

Take a journey with us to see how we discovered that ragwort toxicity could be greatly reduced by natural processes – it’s a short tale of dry tussocks, ant tents and aphid farming… 

When you’re trying to rewild a new site, there are a few things you’ll want to do, according to our Nature Restoration Specialist, Neil:  

“Fields tend to be very flat; especially if they’ve been farmed or mown for a long time, and that’s not great for biodiversity. You want a lot of lumps, which usually get created over decades or even hundreds of years by animals like ants. You might think that grass tussocks are formed from grass, but actually when leaves break down, they leave very little behind, and it’s the movement of soil by ants, or trampling by low densities of livestock that creates this nice lumpy patchwork of tussocks.” 

Ant hills in the Forest of Dean

Over many years, ant hills form a tussocky landscape, like this one

Tussocks create microhabitats

These tussocks are ideal for establishing wildflowers and insects, as they have a warm, sunny southern side, a cold, dark northern side, a dry, free-draining top and soggy edges. This collection of ‘microhabitats’ means a huge variety of different species can take hold here. But how do we get the tussocks established? The team came up with the idea that these dense patches might form when nutrients like horse dung were spread unevenly across the landscape.

How do tussocks form?

Neil had a look into this dung-spreading idea, but, while it might create some tussocks, he didn’t think that it would encourage ant hill formation; 

“Ants don’t tend to build nests in really dense vegetation – when a badger has dug into an ants’ nest and you look inside, you’ll see very few leaves. But it made me more curious about how we could get these tussocks to form – I became interested in finding out how we might encourage ants onto a rewilding plot, and, when I was out on a walk later that week, I noticed a ragwort plant with soil running up its stem.” 

Ants forming a satellite 'tent' colony on ragwort

This ant tent has formed at the base of a ragwort plant

Ant tents on ragwort plants

What Neil had discovered was that ants build little satellite colonies or ‘tents’ away from their main colony, when they find a reliable source of food. You may have seen these tents before – they’re thin lines of soil that snake up a wall or plant stem, seemingly attached by sheer willpower alone. But what could the ants have found in these ragwort plants that would cause them to found a tiny ant village? Neil did a bit of digging online, and found a remarkable story; 

“There are these aphids – Aphis jacobaeae – which live on the ragwort, by feeding on its sap. They in turn are ‘milked’ or ‘farmed’ by the ants, which come along every so often and remove the sugary excretions that the aphids produce. These excretions get taken back to the ant colony for food  – they’re actually just Black Garden Ants, Lasius niger, that you might see in your lawn. And how does the Ragwort feel about this? Well, the ants protect it against other herbivores, including its worst enemy – the Cinnabar moth caterpillar. You’ve probably seen these yellow and black caterpillars climbing all over Ragwort if you’ve ever seen a Ragwort plant before – they’re such a common pest. The ants defend the plant by biting the caterpillars relentlessly until they fall off the leaves.” 

Cinnabar moth caterpillars feeding on ragwort

Cinnabar moth caterpillars absorb the toxins from their host plant, making them toxic

Ants protect ragwort from attack

A detailed study, published in 1991 found that the relationship between ants, aphids and ragwort was so strong that the plants produced significantly less toxins when they were infested with aphids. If ants were around, the toxins weren’t required, as ragwort didn’t need to protect itself against Cinnabar moth caterpillars. This effect was so strong that even offspring of plants which had been nibbled by aphids retained their lower levels of toxins. So, an entire population of Ragwort could get established in an aphid-infested field with very low levels of alkaloid toxins, making them much safer for horses. 

Working with nature

So, what does this mean for horse owners? Perhaps the reason that we so often see ragwort infested with cinnabar moth caterpillars is that this beneficial relationship between ants, aphids and the plant isn’t allowed to develop. Allowing the edges of horse fields to ‘go wild’ encourages the formation of ant hills and the resulting aphid/ant relationship could reduce the toxicity of any ragwort plants that do get established in your field.  

It’s worth noting that the use of herbicides will damage the biodiversity of your field, which affects its ability to support populations of many insects, including ants. However, while the effects of aphids are significant, the reduction in toxins recorded in the 1991 study was approximately 50%, so the risk to horses is not eliminated, and hay from the field may still contain alkaloid toxins at low levels. This should take its risk to manageable levels, and allow horse owners to focus on much more potent non-native threats like sycamore, cherry laurel and boxwood, or seasonal dangers like acorns and yew berries. 

Practical advice for landowners – get in touch to find out more: 

  • Try to avoid using herbicides and pesticides 
  • Ragwort toxicity increases as ant/aphid populations decrease 
  • Repeated mowing of fields will reduce biodiversity 

Further reading

Vrieling, Klaas, Wouter Smit, and Ed van der Meijden. “Tritrophic interactions between aphids (Aphis jacobaeae Schrank), ant species, Tyria jacobaeae L., and Senecio jacobaea L. lead to maintenance of genetic variation in pyrrolizidine alkaloid concentration.” Oecologia 86.2 (1991): 177-182. 

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